Method of making a compact fluid coupler for thermal inkjet print cartridge ink reservoir

ABSTRACT

A method for joining two materials together to form a compact leak-resistant seal, particularly suited for use in thermal inkjet print cartridge ink reservoirs. The seal employs a shrink fit to clamp the two materials together. The method includes the steps of forming the first material into a desired first shot structure, placing the first shot structure in a second shot mold, and injecting the second material into the mold under adequate pressure and at an appropriate temperature. The second material has a high degree of mold shrinkage. Upon cooling, the second material shrinks tightly onto the first shot structure to form a molded joint which keeps air out and ink in when wetted and during normal shipping, storage, and operating conditions.

BACKGROUND OF THE INVENTION

This invention relates to ink reservoirs for thermal ink jet ("TIJ")print cartridges.

TIJ technology is widely used in computer printers. Very generally, aTIJ includes a print head typically comprises several tiny controllableink jets, which are selectively activated to release a jet or spray ofink from an ink reservoir onto the print media (such as paper) in orderto create an image or portion of an image. TIJ printers are described,for example, in the Hewlett-Packard Journal, Volume 36, Number 5, May,1985, and Volume 39, Number 4, August, 1988.

In TIJ pens it is necessary to connect the ink reservoir to the printhead. The size of this connection affects the design of the printer thatthe pens are used in. An ideal reservoir-to-print-head coupler, from aprinter design point of view, would be no longer than the TIJ head islong, and would be high or tall enough to allow the drive and pinchwheels to get as close to the print head as possible. Any increase inthe size of this coupler will compromise the paper handling ability,which may affect the print quality, and increase the size of theprinter. Smaller printers are desirable as they conserve desk space andthe materials from which the printer is fabricated.

An intended application for this invention is for a spring bag TIJ pen,although it is not limited to the spring bag pen. In one exemplaryspring bag pen design, the pen frame made of a first molded material islined with a second molded material, such as polyethylene, on the insideto produce a surface suitable for staking the films of the spring bag.The first molded material from which the frame is made could be, forexample, an engineering plastic, and provides the necessary structurefor the pen which could not be accomplished with the second moldedmaterial. This invention relates to the fluid connection of the firstand second molded materials in such a way as to provide aspace-efficient, leak-resistant connection.

Conventional methods of connecting materials include the use of glue,seals, such as gaskets or 0-rings, or mechanical press fits. In thesecases two or more separate parts are fabricated and assembled togetherto form a single unit. Each part must be designed and sized with respectto its needs in manufacturing, structural integrity, and with thetolerance of the mating part in mind. Such joints as these take up muchmore space than joints fabricated in accordance with this invention. Inaddition to taking up much space, the traditional methods produce ajoint whose reliability can be affected by the part tolerances, surfacefinishes, and the assembly operation. The method of this inventionprovides a joint which is less susceptible to surface finish defectsthan joints obtained by such traditional methods.

SUMMARY OF THE INVENTION

A thermal inkjet print cartridge ink reservoir in accordance with theinvention is characterized by a compact, leak-resistant joint betweenfirst and second moldable materials which define the frame of thereservoir. The reservoir includes a first frame element having a snoutend and defining an interior standpipe member through which a channelopening extends. The channel opening extends between the ink reservoirchamber and a thermal inkjet print head. The first frame element isfabricated from a first moldable plastic.

The ink reservoir includes a second frame element fabricated from asecond moldable plastic material characterized by a shrink rate as thematerial cools from a molten state. The second frame element is formedby injection molding and surrounds the periphery of the standpipe memberto thereby provide the compact, leak-resistent joint, in that the secondmoldable material has shrunk about the periphery of the standpipe memberto define the joint.

A method in accordance with this invention is for forming aleak-resistant joint between first and second moldable materials, andincludes the following steps:

molding the first material into a predetermined first shot structuredefining an interior fluid standpipe through which a channel openingextends;

positioning the first shot structure in a second shot mold;

injecting the second moldable material in a molten state into the moldas a second shot wherein the second material surrounds the standpipemember, the second material characterized by a shrink rate as thematerial cools; and

permitting the second material to cool, whereupon the second materialshrinks about the periphery of the standpipe, thereby forming aleak-resistant seal between the first and second materials about thestandpipe.

Use of this method to join the two materials allows the surface of thefirst shot to be used, as molded, and the molding negates the effects ofthe tolerance and surface finish of the second molded material on thejoint. When the second molded material is molded onto the first moldedmaterial it shrinks as it cools and produces a tight joint.

This method of connection is more reliable than conventional methods.Since the second molded material, e.g., polyethylene, is molded onto thefirst molded material, which can be used as a structural element, thefirst molded material imparts stiffness to the second molded material.The second molded material therefore can be designed to be thinner incross section than if the part were made by conventional methods.Because the second molded material is never handled as a separate parton an assembly line, as would be the case in a traditional two-partdesign, its cross sections are not burdened by the stiffness thathandling would require, and therefore the design is more compact fromthis perspective also.

BRIEF DESCRIPTION OF THE DRAWING

These and other features and advantages of the present invention willbecome more apparent from the following detailed description of anexemplary embodiment thereof, as illustrated in the accompanyingdrawings, in which:

FIG. 1 illustrates a thermal inkjet print cartridge ink reservoirembodying the present invention.

FIG. 2 is a close-up view of the snout region of the rigid engineeringplastic member comprising the ink reservoir of FIG. 1.

FIG. 3 is a close-up view of the snout region of the ink reservoir ofFIG. 1, showing both the rigid plastic member and the polyethylenemember comprising the ink reservoir.

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 2.

FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 3.

FIG. 6 illustrates the second "shot" process in which the polyethylenemember comprising the frame of the ink reservoir is molded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1-6 illustrates a compact thermal inkjet print cartridge inkreservoir 20 in accordance with this invention. In this exemplaryembodiment, the backbone or frame 22 of the reservoir 20 comprises twochemically dissimilar plastics, an engineering plastic, e.g., aglass-filled modified polyphenylene oxide (such as the material soldunder the trademark "NORYL"), and 10 percent glass-filled polyethylene,which are molded one onto the other to form a leak-resistant joint inaccordance with this invention. The frame 22 is stiffened by a pair ofsheet metal covers 24 (only one of which is visible in FIG. 1) whichattach to its sides. This structure of frame 22 and covers 24 isintended for use with a spring bag ink delivery system of the typedescribed in pending U.S. patent application Ser. No. 07/717,735, filedJun. 19, 1991, now U.S. Pat. No. 5,359,353 entitled "Spring-Bag PrinterInk Cartridge with Volume Indicator," by David S. Hunt and W. Bruce Reidand assigned to a common assignee with the present invention. A printhead (not shown) is connected at the snout end 26 of the reservoir forselectively releasing a jet of ink from the reservoir.

In accordance with the invention, the seal between the two dissimilarmaterials comprising the frame 22 employs a shrink fit at the containersnout end to clamp the two plastics tightly together. FIG. 2 illustratesthe snout end 26 of the reservoir 20 in its form prior to molding thepolyethylene 22B onto the engineering plastic frame member 22A. Themember 22A is an integrally formed frame member molded of theengineering plastic. At the snout end 26, the member 22A defines aninterior, upright fluid standpipe 28 having an interior opening 30defined therein which extends through the standpipe to an opening formedin the exterior surface of the frame member 22A. It is through thisopening that the ink will flow from the reservoir. The ink jet head (notshown) will be positioned along the surface 36.

The end 26 of the member 22A is shown in further detail in thecross-sectional view of FIG. 4. FIG. 2 illustrates the open region 32surrounding the upwardly extending fluid standpipe 28 within the frame22A. A pair of spaced ribs 34 and 35 protrude from the exterior side ofthe standpipe 28.

Now referring to FIGS. 3 and 5, the reservoir frame 22 is shown with thepolyethylene layer 22B molded to the inside surface of the frame 22A.The layer 22B in the region 26 is best illustrated in thecross-sectional view of FIG. 5. The polyethylene material 22B has beenmolded around the periphery of the standpipe 28, without covering theopening 30. The polyethylene material 22B provides a surface to whichthe spring bag film may be staked. It is therefore important that therebe no leaks between the standpipe 28 and the surrounding molded layer ofpolyethylene, as this would defeat the integrity of the reservoir, andpermit ink to leak from the reservoir, or air to get into the reservoir.

The method of molding the layer 22B to the frame element 22A is nowdescribed. First, the frame element 22A of modified polyphenylene oxide,i.e., a first molded material, is fabricated in a plastic injectionmold. This part 22A, referred to as the "first shot," is illustrated inFIGS. 2 and 4. The first shot 22A is next inserted into a second mold,where the polyethylene 22B, i.e., the second molded material, is moldedonto it. The polyethylene is injected into the mold under appropriatepressure and at an appropriate temperature. For polyethylene, anexemplary pressure is in the range of 4,000 to 10,000 psi, and anexemplary melt temperature is 400° F. This polyethylene "second shot"has a degree of mold shrinkage (such as, for high density polyethylenewithout glass, about 0.022 inches/inch); upon cooling, the polyethylenematerial shrinks tightly onto the ribs of the first shot. Thus, thenecessary property of the second molded material is that it shrinksduring the cooling process.

FIG. 6 illustrates the second shot molding process. A cavity mold pin 50is extended above the top of the channel 30. The cavity mold pin 50 isattached to and moves with the mold half 54. A channel cavity mold pin52 is inserted into the channel opening 30 formed in the frame member22A and against the cavity mold pin 50. The mold halves 54 and 56 areclosed together, defining the interior opening into which the moltenpolyethylene is injected or "shot," together with surfaces of the firstmolded material. Thus, according to another aspect of the invention, themold cavity for the second shot is partially defined by surfaces of thefirst shot, i.e., the first molded material. Surfaces of the framemember 22A serve as stop surfaces against which the respective moldhalves bear when the mold is closed to stop the flow of the moltensecond material. Thus, in FIG. 6 surfaces 23A and 23B of the framemember 22A are contacted, or brought in close proximity, such as 0.001inch or less, by corresponding surfaces of the mold halves 54 and 56,and prevent flow of the second material into the mold cavities generallyindicated by 55 and 57.

After the second "shot" of polyethylene is cooled, forming thepolyethylene member 22B, the mold halves 54 and 56 are separated. Themold pin 52 is withdrawn during mold separation.

The molded joint resulting from this invention retains water and thermalinkjet printing inks, and keeps air out under moderate pressure andvacuum, through a range of environmental conditions normally experiencedby office products. The internal stresses inherent to the second shot,as it shrinks about the standpipe, keep it from pulling away from thefirst shot. The mold and first shot material, which are at a lowertemperature than the second shot molding temperature, cool the secondshot material. When the polymer passes through its glass transitiontemperature, it changes phases from liquid to solid. When in the solidstate the plastic temperature continues to drop from its glasstransition temperature T_(g) to the mold temperature, e.g., where T_(g)is on the order of 300° F. and the mold temperature is on the order of100° F. The thermal contractions during this part of the cooling processresults in the formation of internal stresses in the now solid secondshot.

The process of this invention is applicable to multicavity molding andalso two-shot molding, where both plastics are injected during differentcycles of the same molding machine. As is well known in the art,multi-cavity molds are used to produce as many parts per cycle as thereare cavities in the mold.

Pressure decay leak-testing of parts fabricated using this seal showminimal leak rates. Further, the seal has proven to endure throughoutthe print cartridge assembly process, during which the ink reservoir issubjected to mechanical and thermal stresses. The seal has been testedsuccessfully with various ribs and different plastic materials, such aspolysulfone for the first shot, i.e., the first molded material, andglass-filled polyethylene as the second shot, i.e., the second moldedmaterial. Other materials may be suitable for the first and secondmolded materials.

The joint created by the method of the present invention is resistent toair leaks into the reservoir and ink leaks out of the reservoir, i.e.,it is resistent to the leakage of air into the closed ink reservoir viathe joint at the materials interface at the standpipe, and to theleakage of ink out of the interface via the joint. The joint is of valueeven if not air-tight, as it would be necessary for air to bubblethrough the interface formed by the materials of the first and secondshots wetted by ink via a bubble generator effect. Air would bubblethrough the interface only under a pressure differential well aboveconditions likely to be faced by an ink jet cartridge.

While the standpipe described above has two ribs, such ribs are notnecessary for the joint to properly function as a leak-resistent joint.The ribs do add leak-resistent margin by making the capillary path thatthe ink must travel to leak out more tortuous, and therefor adds to theenergy necessary for the ink to leak. However, some applications may notallow the use of such ribs, and the joint without ribs is stillleak-resistent.

While the preferred embodiment has employed dissimilar plastic materialsas the first and second molded materials, that is not necessary toobtain a leak-resistent joint with the invention. In fact, in particularapplications, the same material can be used for both materials, so longas the material is characterized by the property that it shrinks uponcooling from the liquid state to the solid state.

The first shot material is typically characterized by a higher meltingtemperature than the second shot material. The first shot material couldbe compounded or non-compounded relative to the virgin base material. Inthis context a "compounded" material is one in which additives such asglass bead, glass fiber, talc, metal particles, or the like have beenblended with the base material. For example, compounded materialssuitable for use as the first shot material include 20% glass-filledmodified polyphenylene oxide or glass-filled polysulfone. Polyethyleneterephthalate (PET), either filled or non-filled, is also suitable foruse as the first shot material.

The second shot material preferably has a melting point which is equalto or less than the melting temperature of the first shot material,although in some applications a second shot material with a highermelting temperature than the first shot could be used. The second shotmaterial can be compounded or non-compounded material, such asglass-filled or non-glass-filled polypropylene, or the like, or evenglass-filled or non-glass-filled polysulfone.

An advantage gained from this invention is the ability to attach the twoplastic parts without an intermediate assembly step. Each plasticmaterial is specified for its unique properties in different aspects ofprint cartridge reservoir assembly and operation; previously such acombination had to be fabricated separately and then joined. Using thisseal, no assembly equipment is required, only one part need be handled,and there is no yield loss associated with imperfect joints. The twoplastic components are attached to each other without resorting todevices such as snap fits, screw holes, etc., which would take upadditional space; instead, all of the space in the ink reservoir isutilized for attachment of the covers and the spring bag films, ensuringadequate stiffness and making effective use of the available space forstoring ink.

It is understood that the above-described embodiments are merelyillustrative of the possible specific embodiments which may representprinciples of the present invention. Other arrangements may readily bedevised in accordance with these principles by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. A method for forming a compact, leak-resistantjoint between first and second moldable materials which define the frameof an ink-jetprint cartridge, the first material comprising a moldablerigid plastic and the second material comprising a moldable plasticcharacterized by a shrinkage rate as said second material cools,comprising a sequence of the following steps:molding said first materialinto a predetermined first shot reservoir frame structure defining at asnout end thereof an interior standpipe member through which a channelopening extends, said channel opening for carrying ink in a liquid statefrom an ink reservoir to a printhead region of said cartridge;positioning said first shot structure in a second shot mold, said moldcomprising first and second mold halves which can be closed about saidfirst shot structure and define a portion of one or more mold cavitiesinto which said second material is injected under pressure, and whichcan be opened upon cooling to permit removal of a molded structuretherefrom, wherein a portion of said first shot structure also defines aportion of said one or more mold cavities, and wherein the second shotmold is provided with a channel mold pin for extending through saidchannel opening to prevent said second material from entering saidchannel during a second shot molding step, and with a cavity mold pinwhich extends across said channel opening, said channel mold pinstopping against said cavity mold pin; injecting said second moldablematerial in a molten state into said mold as a second shot wherein saidsecond material surrounds said standpipe member, said second materialhaving a shrinkage rate as said material cools, and wherein said firstshot structure comprises one or more shutoff surfaces against which saidmold halves bear when closed about said first shot structure, andthereby shuts off a path for said molten second moldable material toflow when injected into said mold; permitting said second material tocool, whereupon said second material shrinks about the periphery of saidstandpipe member, thereby forming a leak-resistant seal between saidfirst and second materials about said standpipe, said seal resistant toleaks of said liquid ink out of said standpipe and resistant to airleaks through said seal into said reservoir.
 2. The method of claim 1wherein said first shot structure is characterized by one or moreprotruding peripheral ribs extending outwardly from said standpipemember, wherein said second material is molded about said one or moreribs.
 3. The method of claim 1 wherein said first material is a modifiedpolyphenylene oxide, and second material is a glass-filled polyethylene.4. The method of claim 1 wherein said first material is polysulfone andsaid second material is glass-filled polyethylene.